The 366 daily episodes in 2014 were chronological snapshots of earth history, beginning with the Precambrian in January and on to the Cenozoic in December. You can find them all in the index in the right sidebar. In 2015, the daily episodes for each month were assembled into monthly packages (link in index at right), and a few new episodes were posted from 2015-18. Beginning in May 2019, I'm adding short entries to the blog (not as podcast episodes, at least not for now, sorry!) mostly taken from the Facebook Page posts. Thanks for your interest!

Thursday, May 30, 2019

Cone-in-Cone



The mineral here is just calcite (even though it’s mostly almost black), but it shows interesting features. Cone-in-cone structures are nested cones, seen here in cross section. The inset shows them a little better – in the main photo, they are represented by very narrow vertical triangles.

It’s not certain how these things form, but some kind of systematic displacement because of microscopic crystal growth variations is probably the favored idea. The variations might be because of clay content (which in my specimen might help account for the dark color), or because of changes in volume when aragonite (chemically identical to calcite, calcium carbonate, but a different crystal structure with a different volume) changes to calcite which can happen during diagenesis, the process of sediment solidifying to rock.

Cone-in-cone might also result from pressure variations, either before or after the rock becomes solid. Pressure variations that might depend on the clay content could produce micro-fractures in the calcite that make the individual crystalline material slide consistently to make the cones. This more structural interpretation might be supported by the fact that my specimens are from a seam of calcite about 3 or 4 inches thick that was within thicker, stronger rocks.

Bottom line, the features are caused by some kind of microcrystalline displacement, but exactly how this happens is not settled.

This specimen is from near White Sulphur Springs, Montana. Collected in 2004.


—Richard I. Gibson

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